Installing sound-absorbing built-in ceilings in the classroom and laying carpets on the floor will give you good acoustics and reduce much reverberation time. This method is more economical for the construction of new classrooms and the renovation of existing classrooms.

For small and medium classrooms, if using a built-in ceiling whose NRC>0.75, its reverberation time is acceptable. 

Carpet has a certain absorption effect on high frequency noise, but mainly used to reduce the noise made by students. 

if you deliberately arrange furniture such as cabinets and bookcases to divide the large and flat walls, it can also reduce the echo.